(1) Field of the Invention
The present invention relates to a high-frequency power amplifier device to be used for transmissions carried out by a mobile communication device such as a cell phone.
(2) Description of the Related Art
Recently, high efficiency (in power saving) of transmission power amplifiers which consume a large amount of power as well as the miniaturization of batteries has been regarded important in order to realize small, lightweight cell phones which are able to perform communication for extended periods of time.
The power amplifiers to be used for the transmissions carried out by the cell phones are called power amplifier (PA) modules, and GaAs high-frequency transistors, which have excellent high-frequency characteristics and power efficiency, are mainly used as such modules. The GaAs high-frequency transistors include field-effect transistors (hereinafter referred to as “FETs”) and hetero-bipolar transistors (hereinafter referred to as “HBTs”).
The following describes a power amplifier that uses HBTs, with reference to the diagrams and using three examples. Note that the description will be given with the same referential marks provided to the same components (e.g. Japanese Laid-Open Patent Application No. 11-266130).
FIG. 1 is a diagram showing the outline of a conventional power amplifier 125 illustrated as a first example. As shown in FIG. 1, the high-frequency power inputted from an input terminal 106 passes through an input matching circuit 103, and then, is inputted in a front stage HBT 101 and amplified by the front stage HBT 101. The input matching circuit 103 is configured of a shunt inductor 107 and a serial condenser 108 in an order closest to the input terminal 106. Here, a shunt inductor is an inductor connected between a signal line and a ground, while a serial condenser is a condenser connected in series to a signal line.
The high-frequency power amplified by the front stage HBT 101 passes through an inter-stage matching circuit 104, and then, is inputted in a rear stage HBT 102 and amplified by the rear stage HBT 102. The inter-stage matching circuit 104 is configured of a shunt condenser 109 and a serial inductor 110 in an order closest to the front stage HBT 101. Here, a shunt condenser is a condenser connected between a signal line and a ground, while a serial inductor is an inductor connected in series to a signal line.
The high-frequency power amplified by the rear stage HBT 102 passes through an output matching circuit 105, and then outputted from an output terminal 116. The output matching circuit 105 is configured of a microstrip line 111, a shunt condenser 112 and a serial condenser 113 in an order closest to the rear stage HBT 102. A characteristic impedance of the microstrip line 111 is designed to be 50Ω and the serial condenser 113 is used to interrupt direct current.
A collector power terminal 117 is connected to a collector of the front stage HBT 101 via a chalk coil 123. The chalk coil 123 is placed to prevent the high-frequency power from leaking to the collector power terminal 117. A front stage bias circuit 121a which functions as a temperature compensation circuit is connected to a base terminal of the front stage HBT 101. Control terminals 119 and 120 are connected to the front stage bias circuit 121a. 
Similarly, a collector power terminal 118 is connected to a collector of the rear stage HBT 102 via a chalk coil 124. The chalk coil 124 is placed to prevent the high-frequency power from leaking to the collector power terminal 118. A rear stage bias circuit 121b which functions as a temperature compensation circuit is connected to a base terminal of the rear stage HBT 102. Control terminals 121 and 122 are connected to the rear stage bias circuit 121b. 
FIG. 2 is a diagram showing the structure of the conventional power amplifier 125 illustrated as the first example. As shown in FIG. 2, a module substrate 131 is made up of three layers of resin substrates 131a, 131b and 131c. The dielectric constant of the respective substrates is 4.4. On the rear surface of the lowest substrate 131c, the input terminal 106, the output terminal 116, a ground terminal 130, the collector power terminals 117 and 118, and the control terminals 119 to 122 are formed. On the top surface of the top layer substrate 131a, a metal wiring 132 is formed. The input matching circuit 103, the inter-stage matching circuit 104 and the output matching circuit 105 is each formed by the metal wiring 132, a chip condenser 135 and a chip inductor 136. On the lateral surface of a through-hole 137, a metal film is formed, and the metal wiring formed on each of the substrates 131a, 131b and 131c is electrically connected to the metal film when necessary. An HBT chip 133 for which GaAs is used is implemented on a ground electrode 138 formed on the top surface of the top layer substrate 131a. The HBT chip 133 and the metal wiring 132 are electrically connected by a bonding wire 134.
A power amplifier for wide-band CDMA with adjacent channel power ratio (hereinafter referred to as “ACPR”) characteristic of −40 dBc or below and power efficiency (hereinafter simply referred to as “efficiency”) of 43% or above is required under the conditions such as a frequency of 1940 to 1960 MHz, a power voltage of 3.5V and an output power of 27 dBm. In order to meet such characteristics, it is preferable to set 5Ω for the impedance ZL (TR) viewed from the rear stage HBT 102 in the power amplifier 125. More specifically, the length of the microstrip line 111 may be set to 3.5 mm, the capacity of the shunt condenser 112 to 4.5 pF, and the capacity of the serial condenser 113 to 100 pF.
FIG. 3 is a diagram showing a table listing the characteristics of the power amplifier 125 illustrated as the first example. As shown in FIG. 3, when the impedance ZL (PA) viewed from the output terminal 116 of the power amplifier 125 is set to 50Ω as a terminating resistor, the impedance ZL (TR), as an impedance of the output matching circuit 105 viewed from the output end of the rear stage HBT 102, is 5.1−j0.5Ω, 5Ω, 4.9+j0.5Ω when the frequency is 1940 MHz, 1950 MHz and 1960 MHz respectively. When Z0=5Ω is standardized, a voltage standing wave ratio (hereinafter simply referred to as “VSWR”) in the terminal 126 is 1.1 or below in the frequency ranging from 1940 to 1960 MHz. Here, the power amplifier 125 was evaluated under the loaded conditions such as a power voltage of 3.5V and an output power of 27 dBm. As a result, satisfactory characteristics of ACPR characteristic of −42 dBc and efficiency of 45% could be achieved in the frequency between 1940 to 1960 MHz.
In the high-frequency devices, in general, a ratio Δf/f of a frequency bandwidth Δf for a frequency f is an index indicating the width of a band, and it is difficult to average an RF characteristic within the band as the value gets larger. This also applies to the power amplifier 125, and the larger the ratio Δf/f becomes, the lower the efficiency and the ACPR characteristic become. However, the ratio Δf/f is as low as 1%, therefore, the degradation in the RF characteristic within the operating band cannot be perceived in this case.
Next, the power amplifier illustrated as a second example shall be described.
Along with the widespread of data communication between cell phones, it is an urgent need to increase the number of communication channels. The cell phones adapted for overseas frequency bands are commercialized so that the cell phones can be used abroad. Thus, the tendency in which plural frequency bands are used by one cell phone, so-called, the tendency of multi-band in cell phones is expected to increasingly accelerate in the future. One of the powerful methods for realizing low-cost and miniaturization in the manufacturing of the multi-band cell phones is to use a power amplifier adapted to plural frequency bands.
For example, the shared use of a wide-band CDMA terminal, a kind of the multi-band cell phone and a power amplifier in the two frequency bands of 1.7 GHz band and 1.9 GHz band is conceivable. The frequency range of 1.7 GHz band ranges from 1750 to 1785 MHz, and the frequency range of 1.9 GHz ranges from 1940 to 1960 MHz. For such a shared use in the two frequency bands of 1.7 GHz band and 1.9 GHz band, it is necessary to satisfy desired characteristics in the range between 1750 to 1960 MHz.
The power amplifier 125 of the second example makes certain changes to the output matching circuit 105 in order to be adapted to the frequency band ranging from 1750 to 1960 MHz. More specifically, the length of the microstrip line 111 is set to 3.6 mm and the capacity of the shunt condenser 112 is set to 4.7 pF.
FIG. 4 is a Smith chart showing the impedance ZL (TR) when the output matching circuit 105 is viewed from the output end of the rear stage HBT 102 included in the power amplifier 125 illustrated as the second example. As shown in FIG. 4, it is assumed that the impedance ZL (PA) viewed from the output terminal 116 of the power amplifier 125 is set to 50Ω as a terminating resistor. In this case, the impedance is 5−j0Ω in the frequency of 1850 MHz in the center of the band. However, the impedance is 4.6+j1.6Ω in the frequency of 1960 MHz and is 5.6−j1.6Ω in the frequency of 1750 MHz. When Z0=5Ω is standardized, the VSWR in the terminal 126 is 1.5 or below in the frequency ranging between 1750 and 1960 MHz. It thus seems that the dispersion of the impedance ZL (TR) is enlarged due to the expansion of the frequency band.
FIG. 5 is a diagram showing a table listing the characteristics of the power amplifier 125 illustrated as the second example. As a result of evaluation on the high-frequency power characteristic of the power amplifier 125 under the conditions of a power voltage of 3.5V, an output power of 27 dBm, ACPR characteristic of −42 dBc and efficiency of 45% in the center frequency 1850 MHz are obtained. However, the characteristics degrading is perceived as the frequency gets farther from the center of the band, and ACPR characteristic of −39 dBc and efficiency of 40% are obtained in the lower limit frequency (1750 MHz) while ACPR characteristic of −37 dBc and efficiency of 43% are obtained in the upper limit frequency (1960 MHz).
In the case of the power amplifier 125 of the second example, Δf/f is as high as 10.5%, therefore, both of efficiency and ACPR characteristic decrease in the frequency distant from a center value even within the band (Δf=210 MHz). With a general power amplifier used in a cell phone, the characteristics are degraded in the lower and upper limit frequencies when Δf/f is 5% or higher.
Next, the power amplifier illustrated as a third example shall be described.
FIG. 6 is a diagram showing an outline of a high-frequency power amplifier device which includes the power amplifier 125 illustrated as the third example. As shown in FIG. 6, the transmission wave outputted from the power amplifier 125 is outputted from an antenna 145 via an isolator 141 and a duplexer 142. On the other hand, the reception wave inputted from the antenna 145 is inputted to a reception IC 143 via the duplexer 142.
The isolator 141 is an electric component for transferring high-frequency power only to the output side (duplexer 142 side), and is designed in such a way that the impedance ZL (PA), as an impedance of the isolator 141 viewed from the output terminal 116 of the power amplifier 125, does not change even the impedance between the duplexer 142 and the antenna 145 changes. Since the output terminal 116 of the power amplifier 125 is directly connected to the input terminal of the isolator 141, the input impedance of the isolator 141 becomes the impedance ZL (PA).
FIG. 7 is a Smith chart showing an input impedance of the isolator 141 connected to the output terminal 116 of the power amplifier 125 illustrated as the third example. As shown in FIG. 7, the impedance ZL (PA) is 49+j7.8Ω, 50Ω, 49−j7.9Ω in the frequencies of 1750 MHz, 1850 MHz and 1960 MHz, respectively. The VSWR in the terminal 116 is 1.5 or below in the frequency ranging from 1750 to 1960 MHz.
FIG. 8 is a Smith chart showing the impedance ZL (TR) when the output matching circuit 105 is viewed from the output end of the rear stage HBT 102 included in the power amplifier 125 illustrated as the third example. As shown in FIG. 8, the impedance ZL (TR) is 6.2−j2.4Ω, 5Ω and 4.2+j2.2Ω in the frequencies of 1750 MHz, 1850 MHz and 1960 MHz, respectively. When the VSWR in the terminal 126 is standardized to be Z0=5Ω, the impedance is 2 or below in the frequency ranging from 1750 to 1960 MHz.
In the power amplifier 125 of the third example, as compared with the power amplifier 125 of the second example, by using the isolator 141, the VSWR in the terminal 126 increases from 1.5 or below to 2 or below and the dispersion gets larger. This is attributed to the fact that the input impedance of the isolator 141 has frequency characteristic.
FIG. 9 is a diagram showing a table listing the characteristics of the power amplifier 125 illustrated as the third example. As shown in FIG. 9, the power amplifier 125 was evaluated under the loaded conditions such as a power voltage of 3.5V and an output power of 27 dBm. As a result, ACPR characteristic of −42 dBc and efficiency of 45% are achieved in the center frequency of 1850 MHz. However, the characteristics degrading is perceived as the frequency gets farther from the center of the band, and ACPR characteristic of −37 dBc and efficiency of 40% are gained in the lower limit frequency (1750 MHz) while ACPR characteristic of −37 dBc and efficiency of 43% are obtained in the upper limit frequency (1960 MHz). Thus, Δf/f is as high as 10.5%, and since the use of the isolator further expands the dispersion of the impedance, the decreases of efficiency and ACPR characteristic in the frequency distant from a center value is more notable.
In the conventional power amplifier, however, when the frequency bandwidth is narrow, as in the case of the first example, satisfactory ACPR characteristic and efficiency can be achieved. In the case, however, where the frequency bandwidth gets broader and the ratio Δf/f increases, as in the second example, the frequency dispersion of the impedance ZL (TR) gets larger and ACPR characteristic and efficiency decrease to the extent that the desired characteristics cannot be achieved. Moreover, in the case of directly connecting an isolator to the output side of an amplifier for use, as in the third example, the dispersion of the impedance ZL (TR) gets larger and the decrease in ACPR characteristic and efficiency is much more notable.